1. Field Of The Invention
The present invention relates to stabilized, concentrated or diluted emulsions of the oil-in-water (O/W) type exhibiting a pesticidal activity and to a process for making use of the emulsions. More specifically the invention relates to stable O/W macroemulsions which comprise one or more pesticidal active ingredients in the oil phase or additionally, macroemulsions which comprise one or more pesticidal active ingredients in both the oil and water phases, wherein the oily phase is emulsified or dispersed in the water phase by an emulsifying system; and wherein the emulsions are further stabilized by a solid dispersing agent, namely titanium dioxide, which maintains or improves the emulsion stability. It also relates to a suspoemulsion obtained by milling the said emulsion with an additional solid pesticidal substance.
2. Description Of Related Art
In general, an emulsion is the result of dispersing one immiscible liquid in another, and is made relatively stable by means of one or more emulsifying agents, which are usually surface-active agents.
The result is a "significantly stable" suspension of droplet particles of a certain size of a liquid homogeneously dispersed within a second immiscible liquid, defined as the continuous phase. The phase "significantly stable" is a relative one meaning relative to the intended use of the emulsion and to the relative ability of one emulsifying system vs. another to stabilize a given system of various differing components, which are additionally subject to various physical and chemical conditions or factors.
The basic factor in the stability or instability of an emulsion is the degree of interfacial tension (i.e., free energy) between the droplets of the dispersed liquid and the other continuous liquid phase. A high interfacial tension makes an emulsion inherently (basically) thermodynamically unstable.
The purpose of the emulsifying (or dispersing) agent, which is usually a surface-active agent, is to interact at the interface between the dispersed liquid droplets and the other continuous liquid phase. It thus functions to stabilize a basically unstable system by adsorption at the liquid/liquid interface as an oriented interfacial film. The result is a reduction in interfacial tension and a decrease in the rate of coalescence of the dispersed liquid particles by forming mechanical, steric and/or electrical barriers around them.
Emulsions may be categorized in two ways: first by the size of the dispersed particles (that is to say micro- vs. macroemulsions) and second by the nature of which phase forms the dispersed droplets and which is the continuous phase (that is to say oil-in-water O/W, vs. water-in-oil, W/O.
Both O/W and W/O micro- and macroemulsions are used for pesticidal compositions, the preference depending upon the system components and the required stability criteria. Macroemulsions generally have a dispersed droplet particle size from about 0.2 to about 50 microns. However, they are basically more unstable than a microemulsion (&lt;0.2 microns), generally due to a wider particle size distribution whereby there is a higher tendency of larger droplets to coalesce with smaller ones and thus more readily break the emulsion.
The choice between O/W and W/O emulsions also depends upon the system components and the required stability criteria. An O/W is generally produced by emulsifying agents which are more soluble in the water than in the oil phase. The reverse generally provides W/O emulsions.
While the use of emulsions is frequently advantageous, their preparation and maintaining their stability frequently involves substantial experimentation (trial and error) and even when the compositions may have only limited stability in either their concentrated form or in end-use diluted compositions.
The emulsifying/dispersing system thus plays a key role in providing stable emulsions, but it is frequently complex and not easy to identify because of the required optimization of many different properties/characteristics such as the following:
There must be good surface activity to produce a low interfacial tension in the system used. The emulsifier must have the tendency to migrate to the interface rather than remain dissolved in either bulk phase.
The emulsifier must form, by itself or with other adsorbed molecules, a condensed lateral interfacial film.
The emulsifier must migrate to the interface at sufficient rate to reduce the interfacial tension to a low value during the time the emulsion is produced.
The emulsifier is best a mixture of a preferentially oil-soluble surface-active agent and a preferentially water-soluble one. This frequently produces a better and more stable emulsion.
An emulsifier that is preferentially water soluble (generally HLB of 8-18) will generally produce lower interfacial tension (i.e. contact angle &lt;90.degree.) at the water interface and produce O/W emulsions. Hydrophilic groups in the interfacial film provide a barrier to coalescence of oil droplets in O/W emulsions.
A suitable emulsifier for an O/W emulsion should give a PIT (phase inversion temperature) of 20.degree.-60.degree. C. higher than the normal storage temperature of the emulsion.
An emulsifier which inhibits or reduces components of the dispersed droplets from wetting the interfacial film (i.e., high contact angle between emulsifier in the film and the components in the droplet) will provide droplets that will not easily coalesce and thus stabilize the emulsion.
The above properties of a good emulsifying/dispersing system will thus determine the relative importance and influence of the following factors which are generally recognized by one skilled in the art to be important in determining emulsion stability (the resistance of emulsions to coalescence of their dispersed droplets--i.e., "resistance to breaking the emulsion").
The Physical Nature of Interfacial Film--mechanical strength and intermolecular forces.
The Existence of Electrical or Steric Barrier on the Droplets--significant in O/W emulsions.
The Viscosity of the Continuous Phase--reduced diffusion reduces droplet collision and thus reduces coalescense.
The Size Distribution of the Droplets--wider size distribution, especially in macroemulsions, allows larger droplets to coalesce at the expense of smaller droplets.
The Phase Volume Ratios--basic instability, especially in macroemulsions, tends to increase as the volume of the dispersed phase increases and the continuous phase decreases.
The Temperature--variation of temperature affects the nature and viscosity of the interfacial film/tension and can inverse or break the emulsion.
From the above discussion, it is thus readily obvious that even for one skilled in the art of emulsion technology, particularly in the area of pesticidal emulsions, the solutions frequently remain complex and each situation may encounter its own unique set of problems which are not necessarily limited to just the stability of the emulsion. Other factors or problems that must be considered include, for example:
Compositions containing more than one pesticidal substance, which differ significantly in chemical and physical properties, in particular where one is soluble in the lipophilic (oil) phase and the other is soluble in the water phase.
The need for improved overall safety characteristics of the composition, for example, by reducing/eliminating organic solvents which are frequently flammable, corrosive or toxic to living systems and are of environmental concern.
Instability resulting during ready to use aqueous dilution of initially stable concentrated emulsions.
For various reasons, including those aspects mentioned above, one may frequently prefer an O/W emulsion. Thus in the specific case of O/W emulsions for pesticidal use where the dispersed oily phase contains a lipophilic pesticidal substance, one or more solvents may be required in the case where the lipophilic substance is naturally in the solid state at the temperature or in the temperature region under consideration. On the other hand, the dispersing phase consists of water, optionally containing a water-soluble pesticidal substance, and a variety of other additives, specifically including surface-active agent(s) which are responsible for the interface between the two phases.
This basic outline, however, is far from enabling to a person skilled in the art to solve all the problems linked with the production of such emulsions in the case of each pesticide.
It is known, in fact, that preformed emulsions of pesticidal lipophilic substances in aqueous media tend to break when, as a result of a temperature variation, these substances change from the solid state into the liquid state, to return into the solid state (solidifying/melting).
This disadvantage is particularly detrimental when the melting points of such pesticidal substances are in the range of temperature variation within which the said substance is stored, because this makes the composition unsuitable for later use.
Similarly, it is known that in the case of pesticidal products which have a melting point below 100.degree. C. it is very difficult to produce an aqueous suspension, because they begin to change state well before their melting point, and this consequently makes them difficult to mill. This is the case especially in hot countries, or in the summer in temperate regions.
These situations are further complicated when, in addition to the pesticidal substance in the oil phase, it is also desired that the composition contains another pesticidal substance soluble in the aqueous phase. This problem, in part, may result due to the tendency of the emulsion to break down because the water soluble pesticide may in itself behave as a surface-active agent, especially in the case of water soluble salts of these compounds. Their solubility in the aqueous phase then causes problems at the interfacial film between the oil and water phases. The result is that the dispersed oil droplet phase has an increased tendency to freely migrate/diffuse into the aqueous phase which leads to coalescence of the oil droplets and instability of the emulsion. Other aspects of the problem may include when is known as the salting out effect caused by high concentration of ions, especially inorganic ions, in the aqueous phase which causes the oil phase to separate out.
While it is known that O/W emulsions can be produced with some very specific systems where a pesticidal substance is only in the oil phase or optionally both the oil and water phases each contain a pesticidal substance, for example, as described in U.S. Pat. Nos. 4,810,279; 4,822,405; 3,873,689; 4,594,096; or in EP 70702 (apparently corresponding to U.S. Pat. No. 4,440,562), the technology is still unpredictable. This unpredictability may be seen, for example, in U.S. Pat. No. 4,853,026 which describes an initially formed O/W emulsion which surprisingly and rapidly inverts to W/O; GB 2,022,418A which only provides a W/O emulsion; EP 289,909A2, in which the examples demonstrate the critical nature and concentration of all the composition components--even slight changes outside the optimum concentrations produced a number of unstable O/W emulsions; or GB 2,082,914A which specifies that a very narrow size distribution is required for the dispersed oil droplet particles.
While the above emulsions, which unpredictably may or may not be stable, utilize well known and recognized ionic or non-ionic emulsifying/dispersing agents, which are surface-active agents, other persons have attempted by less known techniques to provide emulsification by solid powders. These may be present with or without other normal surface-active emulsifiers. Such powders may alternatively be referred to as dispersing or stabilizing agents. For example, unpredictably O/W or W/O emulsions are produced depending upon the interfacial contact angle provided by a specific powder, the nature of the oil and water components, the type of surface-active agents present, the nature of the surface of the powder, and the pH of the system, etc. Schulman et al., Kolloid-Z. 136, 107-119 (1954), reports emulsions stabilized by barium sulfate or co-precipitated barium sulfate-zinc sulfide powders. Scarlett et al., J. Phys. Chem., 31, 1566-1571 (1927) describes emulsification by a number of different powders including glass, copper, pyrite, zinc, charcoal and mercuric iodide. The type of emulsion produced, W/O or O/W, is highly variable and neither of these references describes compositions containing pesticidal substances.
In general, metals and metal oxides, including titanium dioxide, are known to be used in some types of pesticidal/agricultural formulations, for example as described in: DT 3004-010; DT 3005-016; DT 2804-141; J5 5020-750; J6 2004-210A; US 4,493,725; J5 8177-902A; J5 6086-105; or J5 6152-401. These compositions are, however, predominantly granular solids, powders, pastes or creams. In these applications, the function of the inorganic minerals, including that of titanium dioxide is typically as a pigment, a support or carrier, a controlled release agent, an anticaking agent, an antistatic agent, or an acid neutralizing agent.
Only in a few instances has titanium dioxide been disclosed for use in liquid pesticidal compositions, including emulsions. Here again, the purpose of titanium dioxide is usually as a pigment or a support. For example, U.S. Pat. No. 3,873,689, as mentioned previously discloses O/W emulsions. These emulsions contain inert white pigments, such as titanium dioxide, which are typically used at high concentration levels to provide a marking effect during spraying of end-use compositions.
While GB 213,116A does not describe emulsions for agrochemical/pesticidal uses, it does describe pharmaceutical, cosmetic or food emulsions which are W/O and which are stabilized by hydrophobic surface modified metals or metal oxides. The reference broadly discloses metal oxides including that of titanium, but only exemplifies silicas. The examples show that these hydrophobic surface modified suspending agents produce only W/O emulsions. However, on the other hand, while non-surface modified hydrophilic agents initially gave O/W emulsions, these were not stable and rapidly separated into three phases.
Furthermore, while EP 342,134So. Af. 89-3391, corresponding to copending U.S. Ser. No. 07/526,776, filed May 17, 1990 now U.S. Pat. No. 5,076,711 describes O/W emulsions which are stabilized by a titanium dioxide dispersing agent, these emulsions contain only a lipophilic soluble pesticidal substance. These emulsions don't contain any aqueous soluble pesticidal substances, which as discussed above, have a tendency to lead to unstable emulsions.
It is clearly obvious from the above that this emulsion technology is very complex, very specific and thus very unpredictable. Even extensive experimentation may not solve the numerous problems which exist and may not identify a simple uniform (widely applicable) solution to the problem(s), irrespective of the nature and concentration of the pesticidal substances, other adjuvants, processing conditions, and storage and application conditions of both concentrated and ready to use diluted O/W emulsions.